Single crystals are useful in a wide variety of applications, including, for example, electronic and optoelectronic devices, lasers, and optics. Crystal performance in these applications often is limited by the material quality of the crystals, in particular, the concentration of defects. Important classes of defects include voids, micro-pipes, nano-pipes, dislocations, interstitials, and vacancies. Defects also introduce strain into crystals, which tends to degrade the quality, performance, and lifetime of epitaxial layers and electronic devices grown on wafers sliced from these crystals Reduction or elimination of these defects during crystal growth can be very difficult, so alternate means would be useful. Defects can also exist in amorphous materials, such as glasses, plastics, and metals, which may detract from their optical, mechanical, electrical, or visual properties.
In the case of silicon carbide (SiC), large area single crystal wafers for electronic device applications are available commercially in several polytypes (6H, 4H, 15R). However, these wafers typically contain dislocations at a concentration of 104-106 cm−2 and micro-pipes at a density of 10-100 cm−2. Defect concentrations tend to be higher in semi-insulating SiC, suitable for high-frequency device applications, than in n-type SiC. Both micro-pipes and dislocations have been strongly linked with device failure and degradation phenomena. One reason why silicon carbide based devices have not emerged from small niche markets into large scale, high-power applications is the unreliability of devices resulting from substrate defects. Other than US Patent Publication No. 2002 0059901A1 which discloses a method for growing SiC crystals with a micropipe density below 10 cm−2, Applicants are not aware of any currently available method to grow SiC crystals with a micropipe density below 1 cm−2, or SiC crystals of at least 25 mm, at least 50 mm, at least 75 mm, or even at least 100 mm diameter that are entirely free of micropipes.
There are several references in the prior art disclosing methods for covering over micropipes in SiC or filling micropipes near the surface. For example, U.S. Pat. No. 5,679,153 discloses a liquid phase epitaxy method to cover up existing micropipes in a wafer. U.S. Pat. Nos. 6,214,108 and 6,217,842; and US Patent Publication No. 2002 0069818A1 disclose methods to provide a coating on a micropipe-containing SiC wafer and perform a heat treatment, causing up to 75 μm or more of the micropipe to be closed. However, these methods require expensive procedures to be applied one-at-a-time to individual SiC wafers. Additionally, they do not provide SiC crystals of at least 25 mm, at least 50 mm, at least 75 mm, or at least 100 mm in diameter that are free of micropipes and microvoids through their entire volume.
In the case of gallium nitride, the quality of substrates currently is low, as they are typically prepared by heteroepitaxy on non-GaN substrates, such as, for example, sapphire or silicon carbide. Due to lattice- and thermal-expansion mismatch, the GaN so formed suffers from threading dislocations at concentrations between about 5×105 and 108 cm−2. GaN also tends to grow with substantial concentrations of native defects, such as vacancies, both on Ga and N sites. Simple high-temperature annealing of GaN is precluded by decomposition to Ga+—N2 at temperatures above about 750° C.
The quality of gallium arsenide and indium phosphide wafer substrates is considerably higher than that of SiC or GaN, but the concentration of defects (dislocations, vacancies) is nonetheless sufficiently high to have a deleterious effect on the performance of electronic devices fabricated thereon. The growth technology is sufficiently mature that significant incremental improvements are unlikely. Simple high-temperature annealing is precluded by volatilization of arsenic or phosphorus, which would form vacancies and even droplets of Ga or In.
Banholzer, et al. in Published Patent Application No. WO0213958 disclosed a method to increase the toughness of synthetic diamond by annealing at high pressure and high temperature. There was no specific teaching about reduction in the concentration of voids, micro- or nanopipes, dislocations, or vacancies. Anthony, et al. in Published Patent App. No. WO0213958 and Vagarali, et al. in US Patent Application No. 20010031237A1 disclose modification of the color of natural diamonds by annealing at high pressure and high temperature. These treatments modify the nature, concentration, and distribution of point defects (vacancies, interstitials, substitutional impurities), but these authors provide no teaching about reduction in the concentration of voids, micro- or nanopipes, or dislocations. Webb, et al. in J. Mater. Res., Vol 10, No. 7, p. 1700 (1996), proposed annealing of synthetic type I diamond crystals at 1200°-1700° C. and 50-60 kbars to induce aggregate-nitrogen disassociation and metal coalescence, as well as heal diamond lattice dislocations.
There are several references disclosing annealing crystals in a gas pressure apparatus. U.S. Pat. No. 6,329,215 describes annealing GaN, AlN, and InN under a high nitrogen pressure between 0.1 and 2 GPa as the pressure medium, requiring the use of a very specialized and hazardous gas pressure apparatus. The use of high nitrogen pressure herein is to inhibit decomposition at high temperature, and not to cause defect removal. The annealing conditions taught by Porowski, et al. have been sufficient to anneal point defects and cause atomic diffusion, but no reduction of threading dislocations, micro- or nano-pipes or voids was demonstrated. U.S. Pat. No. 6,447,600 discloses a method for annealing of Si, GaAs, InP, ZnS, and ZnSe in an inert gas at pressures up to 0.3 GPa. However, no reduction of threading dislocations, micro- or nano-pipes or voids is demonstrated and that vacancies in Si could be aggregated into voids. Japanese Patent Publication Nos. JP 10114533A2 and JP 02124729A2 disclose annealing glasses in a gas pressure apparatus at a maximum pressure of 0.2 Gpa.
Thus, there exists a need in the art to heal defects in SiC, GaN, and similar crystals to improve performance of electronic and optoelectronic devices fabricated thereon. In the case of crystals for non-linear optical applications, undesired light scattering will be reduced and laser damage thresholds will be increased. In the case of piezoelectric and relaxor ferroelectric crystals, annealing will increase breakdown fields, efficiency, and lifetime. There similarly exists a need in the art to heal defects in amorphous glasses, plastics, and metals.